Current gas-sweetening technology requires plants to treat
at least 1 million standard cubic meters per day
(MMm3/d) of gas to be economically feasible. This
article presents a versatile technology that can be profitably
applied to any gas flow from 10 thousand standard cubic meters
per day (Mm3/d) up to several million with the
benefit of reducing setup periods. This technology (Fig. 1) has been proven
over five years in a gas-compression plant where sulfur and
mercaptans (RSH) contamination unexpectedly rose from 150 ppm
to 1,000 ppm. This allowed the company to comply with an
existing commercial agreement for 100 Mm3/d.

The problem.

In the proven installation, gas-flow production is collected
along 50 km in an 8-in.-diameter pipeline. Gas-flow temperature
is generally around 60°F, and pressure is as low as 3
kg/cm2. The contaminants to be scavenged are
hydrogen sulfide (H2S), RSH and carbon dioxide (CO2).
Plant process is completed by a set of alternative compressors,
a distillation plant and dew-point
adjustment for high pressure. Afterward, sweetened natural gas
is dispatched through a transport pipeline injection (95%) and
liquefied petroleum gas (LPG) to trucks (5%). The original
sweetening processing plant was designed through seven towers
containing a solid chemical scavenger-based in
OxFey.

Over time, H2S and CO2 levels strongly
rose, and mercaptans appeared as contaminants.

Scavenger products used were specific for H2S
acid treatments, since contamination was prevalent in the
beginning. However, the scenario changed to have lower yields,
requiring using different liquid chemical injections downstream
to obtain the required output specification. Operating costs
increased to a point where the alternative to interrupt supply
needed to be evaluated, despite the reputation impact in the
market.

Due to this problem, it was proposed to test the chemical
process, changing the chemical scavenger, in a
pilot plant. Aminetechnology was quickly discarded
since the gas standard flow being treated was heavy in
contamination, implying an expensive installation and high
operative costs. As a consequence, testing began on a process
using a strong alkali that reacts with weak acid contaminants
forming a buffer solution, which means the possibility of
obtaining a controlled pH in the effluents. Moreover, it is
possible to add commercial value to all the salts obtained in
the reaction.

The design.

A specific study must include a gas-contaminant assessment
to define the technical application in function of the
treatments to byproducts. In the opposite extreme, it is
possible to simply install modular plants just near well-outs.
An important advantage is the protection of pipeline
transporting only sweet gas.

Such control is related to the tower and packed design,
drain and make-up of the scavenger, with continual checking of
pH values. The solubility of H2S in water is
important in industrial practice, especially in these environmental-awareness times. The
fate of such an unfriendly component such as H2S is
important to track. H2S is highly toxic, has a
noxious odor and can form harmful-reaction products [such as
sulfur dioxide (SOx)].

The treatment for the solubility in strong bases is the same
as that in acids, but the effect is dramatically different. As
with the case of strong acids, the amount of the molecular
species is dictated by the partial pressure. However, in this
case, the reactions are shifted to the right producing more of
the ionic sulfide species, thus dramatically increasing the
total H2S concentration.

Fig.
2. The
solution.

Case with CO2, as the contaminant.

CO2 is a gas easily soluble in water and the
solubility equilibrium is maintained in agreement with Raoult
Law. Part of dissolved CO2 is converted to carbonic acid, which reacts as weak
acid forming buffer salts. Both salts obtained have important
industrial uses. On the other hand, the sodium carbonate can react, giving back the
caustic alkali in agreement with the following:

Na2CO3 + CaO > CaCO3 +
NaOH

Case with H2S and CO2, as the contaminant.

In cases where both contaminants are present, plant design
must consider that an H2S acid in spite of having
similar Ka as H3CO2, is more powerful as
a reducer. Thus, H2S is the first priority as a
reactive in the presence of a strong caustic like the
scavenger.

When the designer defines basic engineering, spacial
velocity of sour gas and scavenger flow are dependent on the
quantities of acid to be neutralized. In the case presented in
Table 1, the tower design has the capability of neutralizing,
at first, 30% of the H2S being present. Afterward,
it neutralizes up to 70% of H2S and 60% of the total
CO2 present in the original sour gas. Designers can
define the capability of the strong caustic, while waste
solution pH will vary with the byproduct salts present in the
overall solution. HP

The authors

Carlos Alberto
Ortega Peralta is the executive president at
NEUGA SA. He has over 42 years of experience in process
design and plant processes. Mr. Ortega Peralta invented
the Titular Process.

Maria Jose Ortega
Castelán is a manager NEUGA SA and has 10
years of experience in project management. She
received a degree in industrial engineering and an MBA.
Ms. Ortega Castelán is CFA certified.

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